The Science behind the Colours of Fall Leaves

By Melanie Mathieson
Gardening Guru

Fall always promises to be a great season if you enjoy the changing colours of the trees around us. You may ask why are some years good for fall colour, when others are not? Many factors such as soil conditions, weather, genetics, daylight hours and fall temperatures, all contribute to the intensity and display of fall colours. In years when there has been plenty of rain up until the fall, the trees usually retain their green leaves longer than in years when we have experienced drought conditions. In summers that have had severe drought conditions I have seen some trees (usually birches) lose their leaves as early as August. These leaves have not displayed a beautiful fall colour but more of a brown colour with a dry texture, which is different from the usual fall colour display. Warm sunny days with fairly warm nights, in the fall, cause the trees to retain their green colours longer than fall weather that has warm days with cool nights.
The whole process of leaves changing to fall colours is a slow one and begins as the length of the nights increase. This reduction in the daylight hours causes the tree to produce phytochrome. Phytochrome is the chemical that starts the process of dormancy. A layer of cells is produced between the branch of the tree and the leaf stalk. This layer is called the abscission layer and it blocks the passage of water and nutrients (carbohydrates) to and from the leaf. The production of the green pigment, chlorophyll, which is the predominant colour pigment in all plant leaves, throughout the growing season, begins to break down. Without the chlorophyll to colour the leaves green, we begin to see the other pigments such as carotenoids which give the leaves yellow, orange and brown colours. Some trees also have the ability to form another pigment known as anthocyanin in their genetic makeup. Anthocyanin gives leaves a red or purple colour. For anthocyanins to form there must be sugar present so any weather condition that enhances the production and accumulation of sugars in the leaf helps with the intensity of the red colour. Genetics also plays a role in the rate at which leaves change colours when you are looking at the leaves of a stand of trees that are clones of each other. This is a common occurrence in trembling aspen in our area and in the fall, all the trees within a clone of aspen will change colours exactly at the same rate. A neighbouring clone of aspen with different genetic makeup may change at a different rate from the neighbouring clone but all the trees belonging to this clone will also change at the same rate. This is an interesting phenomenon to watch in the forest in the early fall.

Sunny days result in a high production of carbohydrates in the leaf and cool nights help to break those carbohydrates down into sugars. The cool nights also help to keep those sugars in the leaf instead of going to other parts of the plant. When the skies are cloudy and the nights warm, less sugars are produced and more are moved from the leaf, leaving us with less intense colour. As the abscission layer gets bigger it divides into two layers. One layer is protective and forms on the branch. The other is a separation layer and forms on the leaf stalk (petiole). Once both layers form there is not much left to hold the leaf in place and it falls from the tree. A popular myth about fall colour is that we need a frost to produce good fall colour. Killing frosts and freezing temperatures stop the colour change and kill the leaves. So let’s hope the nights stay cool (well above 0º), but not cold.

I hope this column will help to demystify why a wonderful display of fall colour happens some seasons and not in others. So far this year we seem to have a pretty good variety of colours in our region. I encourage you to take a drive around the District and enjoy the colours while they last. Soon enough we’ll have to be raking all those leaves up and it’s nice to think that at one point we really did enjoy them.